1. Field of the Invention
The present invention relates to an image pickup lens and an image pickup apparatus, and more particularly to an image pickup lens appropriate for use with a vehicle camera, a surveillance camera, or the like having an image sensor, such as a CCD (charge coupled device) or a CMOS (complementary metal oxide semiconductor), and an image pickup apparatus having the image pickup lens.
2. Description of the Related Art
Image pickup devices, such as a vehicle camera for taking images around a vehicle, a cell phone camera, a surveillance camera installed for obtaining surveillance images or security purposes, and the like, are known. Generally, these devices include an image pickup lens and an image sensor, such as a CCD or a CMOS, for converting an image formed by the image pickup lens into an electrical signal. Downsizing and increasing of pixel count for these image sensors have been advancing year after year and, along with this, downsizing of image pickup devices has also been advancing. Consequently, a demand for downsizing and performance improvement of image pickup lenses to be mounted in the image pickup devices has also been growing.
In the mean time, for vehicle cameras and surveillance cameras, inexpensive lenses having high weather resistance and usable in a wide temperature range from an open air temperature in a cold weather region to an in-vehicle temperature in summer of a tropical region with a small F-number that allows night time use and high performance have been sought.
As for the image pickup lenses used in the aforementioned application fields, image pickup lenses described in Japanese Patent No. 3723654 (Patent Document 1), Japanese Unexamined Patent Publication No. 2005-024969 (Patent Document 2), Japanese Patent No. 3723637 (Patent Document 3), U.S. Pat. No. 5,682,269 (Patent Document 4), and Japanese Unexamined Patent Publication No. 11 (1999)-142730 (Patent Document 5) are known. The present applicant has proposed a six element image pickup lens usable in the aforementioned application fields in Japanese Unexamined Patent Publication Nos. 2008-276246 and 2009-143092. Patent Document 1 describes a five element image pickup lens with negative, positive, negative, positive, and positive lens arrangement. Patent Document 2 describes a six element image pickup lens constituted by a front group that includes an aspherical lens and a rear group having a positive refractive power. Patent Document 3 describes a six element image pickup lens having a long back focus length. Patent Document 4 describes a six element image pickup lens having a cemented lens on the most image side. Patent Document 5 describes a six element image pickup lens that includes an aspherical lens.
The image pickup lens described in Patent Document 1 has only five elements and there is still room for improvement in aberration correction in comparison with those having six elements or more. The image pickup lens described in Patent Document 2 or 5 uses an aspherical lens. If a plastic is used as the material of the aspherical lens, performance change due to temperature change is likely to occur, thereby causing a problem when the image pickup lens is used over a wide temperature range. Consequently, it is desirable that a glass material is used for the aspherical lens. The use of a glass material for an aspherical lens, however, requires glass molding, resulting in an expensive aspherical lens.
The image pickup lens described in Patent Document 3 or 4 uses only glass spherical lenses, which is advantageous over an image pickup lens that uses a glass mold aspherical lens. The image pickup lens described in Patent Document 3, however, has a total length too long to be qualified as a sufficiently downsized image pickup lens. The image pickup lens described in Patent Document 4 is relatively downsized but has an F-number of 2.8 which is rather too large to be used for vehicle and surveillance applications which are expected to occasionally perform imaging under a low illumination environment.
In view of the circumstances described above, it is an object of the present invention to provide a small and inexpensive image pickup lens, yet having a small F-number and high optical performance, and an image pickup apparatus having the image pickup lens.